Acoustical tunnel or the like detecting device



April 23, 1968 N. A. FROST 3,379,274

ACOUSTICAIJ TUNNEL OR THE LIKE DETECTING DEVIUE Filed Sept. 6, 1966 L 34 lo 24 2s 2 54 30 I so i k I z l 4s 44 J 22 |e- I I2 INVEN'T'OR ,l6'-- J NELSON A.FROST Fj BY .QFWMMJMWQZM ATTORNEYS United States Patent 3,379,274 ACOUSTHCAL TUNNEL OR THE LIKE DETECTING DEVECE Nelson A. Frost, 232 Mill St., Byram, Conn.

Filed Sept. 6, 1966, Ser. No. 577,327 6 Claims. (Cl. 181-.5)

ABSTRACT (3F THE DISCLOSURE This invention relates to detecting devices and more particularly to a device especially adapted for locating hidden tunnels of the type used by guerrillas or insurgents.

In certain areas of hostility, as for example, in Southeast Asia, insurgents or guerrillas have effectively used tunnels in which to hide from their attackers and because these tunnels have been so well camouflaged and disguised it is in many cases extremely difiicult to locate the tunnels and, in fact, it has often been found that an area which has been completely canvassed by the attackers without revealing any of the insurgent forces has actually harbored forces up to regimental size hidden beneath the searchers feet in tunnel complexes which have defied detection.

The object of the present invention is to provide a device which is particularly, though not exclusively, adapted for the location of concealed tunnels.

More particularly, it is an object of the invention to provide an audio detecting device which produces, through impact, a characteristic noise response, more or less h llow in nature, when operated over a tunnel with this noise being distinguishable from a noise response which is produced when the device is applied to solid ground.

Other objects and their attendant advantages will become apparent as the following detailed description is read in conjunction with the accompanying drawings wherein:

FIG. 1 is a vertical cross-sectional view of a device incorporating the features of the present invention;

FIG. 2 illustrates the manner of use of one embodiment of the present invention; and

FIG. 3 illustrates a second manner of use of the invention.

Referring now to the drawings and particularly to FIG. 1, the numeral designates a dome-shaped body member of strong plastic whose interior is recessed to provide a sounding cavity 12. The body 10 is centrally bored to provide a vertical passage 14 slidably containing a striker plunger 16 having a veritcal groove 18 adjacent its upper end to receive the projecting end of a keeper member 20 which is fastened to the upper wall 22 of the cavity 12, as shown, in order to limit the degree of reciprocatory movement of the plunger 16 in the passage 14 as is clearly evident from the drawing.

The plunger 16 is provided adjacent its upper end with a blind radial bore 20 which, when the plunger is in the full line position of the drawing is adapted to receive the rounded outer end of a trigger member 22 which is urged at all times toward the plunger by a spring 24 interposed between the outer end of the trigger member and a stop screw 26 which serves to close the outer end of a passage 28 in which the trigger member 22 and spring 24- operate. Connected to the outer end of the trigger member 22 is an operating rod 30 which extends co-axially outwardly of the passage 28 through a suitably drilled opening in the closure member 26 with the outer end of the rod extending beyond the outer surface of the body 10 and being provided with an eye 32 engaged by a trigger release chain 34, the arrangement being such that when the chain 34 is pulled to move the trigger 22 clear of the blind bore 29 and the plunger 16, the latter is free to move downwardly to the maximum extent permitted by the slot 18, should conditions permit such maximum travel.

Bearing on the upper end of the plunger 16 is a heavy spring 36 whose upper end is received in a hollow plunger 38 having a slot 40 in the side thereof which receives the inner end of a screw 42 which limits the degree of travel of the hollow plunger 38.

The wall 22 of the sounding cavity 12 is recessed at 44 to receive a transistorized audio pick-up microphone 46 which is connected by electrical leads 48 extending through a suitably sealed passageway 50 in the body 10 to an audio sounding or speaker device 52 which may be part of a trigger release handle 53 connected to the chain 34 and held in the hand of the operator as illustrated in FIG. 2. As can be seen in FIG. 1 the leads 48 are desirably intertwined among the links of the chain 34 with there being sufficient slack 54 at its lower end to permit operation of the trigger 22 without exerting any strain on the leads.

In use, assuming the striker plunger 16 to be initially in its extended position as indicated by the phantom lines 16 when the operator arrives in a suspected area, he places the body 10 on the ground and immediately puts his foot on the top of the hollow plunger 38 and presses downwardly which has the effect of causing the hollow plunger 33 to move into the interior of the body 10 while simultaneously the striker plunger 16 is urged upwardly until the blind bore 20 registers with the trigger 22 which then automatically snaps into the bore 20 to retain the member 16 in the position of the drawing. The operator then waits until the external noise level has abated sufliciently at which time he pulls outwardly on the chain 34 to release the trigger whereupon the plunger 16 is driven downwardly by the compressed spring 36 until it impacts with the ground. If the ground is solid beneath the detecting device a characteristic thud which the operator has been trained to recognize is delivered by the audio sounding device 52. On the other hand, if the ground beneath the device, even at a depth of several feet below the surface, is hollow, as where a tunnel might be located, or particularly a tunnel entrance, a hollow resonant sound is produced and received by the pick-up 46 with a characteristic audio response being heard by the operator which indicates the possibility that a tunnel exists beneath the area. If such sound is heard, the area is immediately investigated by digging to confirm the presence of a tunnel.

In the event that the characteristic sound of a possible tunnel is not received, the operator will normally immediately recock the plunger 16 by hearing down onto the hollow plunger 32 with increased force which will usually be sufiicient to move or to sink the outer peripheral edge of the body 10 downwardly enough to permit the plunger 16 to move upwardly a sufficient distance for the trigger 22 to re-engage the blind bore 20. If, because of the nature of the ground surface, it is not possible for the device to be recocked merely by stepping hard onto the plunger 38, any flat object, which may be specially constructed such as a small disc, or a rock could be placed under the striker plunger 16 prior to stepping on the hollow plunger 38.

Instead of having a hand-held audio response member 52 as shown in FIG. 2, the operator may be provided with earphones as shown in FIG. 3. The external audio responder 52 shown in FIG. 2 is often preferable to the single earphones 56 shown in FIG. 3 because the external responder permits a consensus of a plurality of trained listener as to whether or not there is likcihood of a tunnel under the device.

t will be apparent that the device of the invention is susceptible of a variety of modifications and changes without, however, departing from the scope and spirit of the appended claims.

What is claimed is:

1. A detector for locating a hollow space beyond a solid appearing surface comprising a body member having a side adapted to be placed in engagement wtih the surface, an open sound-receiving cavity in said side of said body member, a plunger slidable in said body member between extended and depressed positions and including a manually operable part extending outwardly of said body member, a striker member carried by said body member and movable through said cavity between extended and retracted positions, spring means between said plunger and striker member arranged to resist movement of said plunger and striker member to their respective depessed and retracted positions, trigger means in said body member releasably engageable solely with said striker member to retain the same in its retracted position, said plunger being constructed and arranged to be movable to and retained in its depressed position solely by external manual force operating on said outwardly extending manually operabie part, said spring means being stressed by movement of said plunger to its depressed position to drive said striking member from its retracted towards its extended position and into sound producing striking engagement with said surface upon release of said trigger means.

2. The detector of claim 1 including an audio detector microphone position in said body in sound receiving relation with said cavity, and means for audibly externally reproducing sound impinging on said microphone as a result of said striker member engaging the surface of said layer.

3. The detector of claim 2 wherein said sound reproducing means comprises a hand-held speaker device.

4. The detector of claim 2 wherein said Sound reproducing means comprises earphones.

5. The detector of claim 1 including trigger releasing means comprising an operating element connected at one end to said trigger means and extending away from said body member, and means carried by the opposite ends of said element for manual engagement by an operator to effect release of said trigger means when a manual force is exerted on said operating element.

6. The detector of claim 1 wherein said plunger cooperates with said body member to effect retraction of said striker member when the foot of an operator is applied to said plunger means with a predetermined force.

References Cited UNITED STATES PATENTS 1,753,368 4/1930 Dubois-Reymond et a1. 1810.5 2,679,205 5/ 1954 Piety 181-0.5 2,993,554 7/1961 Towell et al. 181-0.5 3,053,220 9/1962 Sawyer 1810.5 3,223,194 12/1965 Michael 1810.5 3,249,177 5/1966 Chelminski 181-05 FOREIGN PATENTS 612,569 4/1935 Germany.

BENJAMIN A. BORCHELT, Primary Examiner.

SAMUEL FEINBERG, Examiner.

W. KUIAWA, Assistant Examiner. 

